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Symptom:
Trial and error tuning usually does not give the best results for the full
range of conditions. Therefore, the loop performance must be sacrificed at
some conditions, or the controller must be retuned which becomes a time
problem.
Probable cause: Traditional tuning
methods result in tuning parameters, gain and reset, which cause some
overshoot and undershoot of the process variable after a setpoint change.
The correct reset (also called integral) setting can eliminate this
deviation between PV and SP.
Corrective action:
Based on the open loop response test, the process time constant is a known
value. The reset speed in the controller should be set to be same time
value in units of time/repeat. In other words, the reset time/repeat has
the same time value as the process time constant.
This
plot shows how a loop would respond if the reset speed is set equal to the
process time constant. Notice how the PV comes back to SP with any
overshoot. A process may have different time constants at different
conditions, which would require retuning the controller with the correct
reset speed. One stipulation in order to achieve this performance is that
the control valve must have a positioner so that it follows the controller
output signal very closely.
Note:
Be cautious of different units of reset in different controller models or
manufacturers. If the units are repeat/time, take the inverse of the
process time constant for the reset setting. |
